1. Field of the Invention
This invention relates to a catalyst for purifying hydrocarbon (HC), carbon monoxide (CO) and nitrogen oxide (NOx) in an exhaust gas discharged from an internal combustion engine for automobiles or the like.
2. Description of the Related Art
As the conventional catalyst for the purification of exhaust gas, there is used a structure that a catalytic metal such as platinum (Pt), palladium (Pd), rhodium (Rh) or the like is carried on alumina, cerium oxide or the like and the coated onto a monolith carrier. Particularly, Pd is cheap and excellent in the catalytic activity at a low temperature, so that it is attempted to prepare various catalysts for the purification of exhaust gas using Pd as a catalyst active site.
In the conventional exhaust gas purification catalysts provided with Pd carried layer, however, exhaust gases widely varying from reducing composition to oxidizing composition are directly introduced into the catalyst to purify harmful components, so that if Pd is carried on a certain kind of a substance, the purification ratio is relatively high in a particular exhaust gas atmosphere, but it is a fear that the purification ratio can not be maintained at a high level in exhaust gas atmospheres other than the above particular atmosphere.